In recent years, with the increasing awareness of health issues, environmental issues and issues of safety on food, methods for detecting substances related to these issues (so-called chemical substances like substance related to living body (hereafter referred to as a target substance)) have been demanded.
A detecting technique with high sensitivity is often required for detecting a target substance. This is because a collectable amount of a sample containing the target substance is often very small, and furthermore, even the sample occasionally contains only a trace amount of the target substance among variously mixed substances, as in the case of protein in blood. For this reason, a technique for detecting the target substance has been demanded which has such a high sensitivity as to be capable of detecting a trace amount of the target substance contained in a trace amount of an analyte.
One technique for responding such a demand is a measurement method with the use of a plasmon resonance phenomenon occurring around a metallic particle, which is being developed.
Japanese Patent No. 3,452,837 discloses a technique of detecting how much substance has been absorbed by or has deposited on a fine metallic particle fixed on a substrate, by using a plasmon resonance phenomenon occurring around the fine metallic particle, measuring the change of an absorption spectrum of a light having passed through a medium, and detecting the change of a refractive index of the medium.
In addition, Japanese Patent Application Laid-Open No. 2005-283556 discloses a technique of measuring a change of an absorption spectrum for a transmitted light at high sensitivity, by projecting measuring light into a narrow duct having a plurality of fine metallic particles arranged therein.
In the technique according to Japanese Patent No. 3,452,837, it is assumed that a substrate having a lower density of fine metallic particles would more easily detect a change of an absorption spectrum, because the measured absorption spectrum shows only the absorption spectrum of a single fine metallic particle and is hardly affected by interaction between the fine metallic particles. However, it is also assumed that when the substrate has a lower density of fine metallic particles thereon, an absolute absorbance of a spectrum decreases, because the number of fine metallic particles per unit area becomes few. When the absorbance of the spectrum decreases, it becomes difficult to measure characteristics of the absorption spectrum, and consequently to stably detect the change of the spectrum at high sensitivity with a detector, which is a problem.
On the other hand, the technique in Japanese Patent Application Laid-Open No. 2005-283556 can increase the absorbance of the spectrum. However, the technique has a problem that it is difficult to produce such an element that gives rise to less interaction between metallic nanoparticles.